Phosphorus is an essential element for the growth of all organisms. In livestock production, feed must be supplemented with inorganic phosphorus in order to obtain a good growth performance of monogastric animals (e.g. pigs, poultry and fish).
In contrast, no inorganic phosphate needs to be added to the feedstuffs of ruminant animals. Microorganisms, present in the rumen, produce enzymes which catalyze the conversion of phytate (myo-inositolhexakis-phosphate) to inositol and inorganic phosphate.
Phytate occurs as a storage phosphorus source in virtually all feed substances originating from plants (for a review see: Phytic acid, chemistry and applications, E. Graf (ed.), Pilatus Press; Minneapolis, Minn., U.S.A. (1986)). Phytate comprises 1-3% of all nuts, cereals, legumes, oil seeds, spores and pollen. Complex salts of phytic acid are termed phytin. Phytic acid is considered to be an anti-nutritional factor since it chelates minerals such as calcium, zinc, magnesium, iron and may also react with proteins, thereby decreasing the bioavailability of protein and nutritionally important minerals.
Phytate phosphorus passes through the gastrointestinal tract of monogastric animals and is excreted in the manure. Though some hydrolysis of phytate does occur in the colon, the thus-released inorganic phosphorus has no nutritional value since inorganic phosphorus is absorbed only in the small intestine. As a consequence, a significant amount of the nutritionally important phosphorus is not used by monogastric animals, despite its presence in the feed.
The excretion of phytate phosphorus in manure has further consequences. Intensive livestock. production has increased enormously during the past decades. Consequently, the amount of manure produced has increased correspondingly and has caused environmental problems in various parts of the world. This is due, in part, to the accumulation of phosphate from manure in surface waters which has caused eutrophication.
The enzymes produced by microorganisms, that catalyze the conversion of phytate to inositol and inorganic phosphorus are broadly known as phytases. Phytase producing microorganisms comprise bacteria such as Bacillus subtilis (V. K. Paver and V. J. Jagannathan (1982) J. Bacteriol. 151, 1102-1108) and Pseudonomas (D. J. Cosgrove (1970) Austral. J. Biol. Sci. 23, 1207-1220); yeasts such as Saccharomyces cerevisiae (N. R. Nayini and P. Markakis (1984) Lebensmittel Wissenschaft und Technologie 17, 24-26); and fungi such as Aspergillus terreus (K. Yamada, Y. Minoda and S. Yamamoto (1986) Agric. Biol. Chem. 32, 1275-1282). Various other Aspergillus; species are known to produce phytase, of which, the phytase produced by Aspergillus ficuum has been determined to possess one of the highest levels of specific activity, as well as having better thermostability than phytases produced by other microorganisms (unpublished observations).
The concept of adding microbial phytase to the feedstuffs of monogastric animals has been previously described (Ware, J. H., Bluff, L. and Shieh, T. R. (1967) U.S. Pat. No. 3,297,548; Nelson, T. S., Shieh, T. R., Wodzinski, R. J. and Wetre, J. H. (1971) J. Nutrition 101, 1289-1294). To date, however, application of this concept has not been commercially feasible, due to the high cost of the production of the microbial enzymes (Y. W. Han (1989) Animal Feed Sci. & Technol. 24, 345-350). For economic reasons, inorganic phosphorus is still added to monogastric animal feedstuffs.
Microbial phytases have found other industrial uses as well. Exemplary of such utilities is an industrial process for the production of starch from cereals such as corn and wheat. Waste products comprising e.g. corn gluten feeds from such a wet milling process are sold as animal feed. During the steeping process phytase may be supplemented. Conditions (T.apprxeq.50.degree. C. and pH=5.5) are ideal for fungal phytases (see e.g. European Patent Application 0 321 004 to Alko Ltd.). Advantageously, animal feeds derived from the waste products of this process will contain phosphate instead of phytate.
It has also been conceived that phytases may be used in soy processing (see Finase# Enzymes By Alko, a product information brochure published by Alko Ltd., Rajamaki, Finland). Soybean meal contains high levels of the anti-nutritional factor phytate which renders this protein source unsuitable for application in baby food and feed for fish, calves and other non-ruminants. Enzymatic upgrading of this valuable protein source improves the nutritional and commercial value of this material.
Other researchers have become interested in better characterizing various phytases and improving procedures for the production and use of these phytases. Ullah has published a procedure for the purification of phytase from wild-type Aspergillus ficuum, as well as having determined several biochemical parameters of the product obtained by this purification procedure (Ullah, A. (1988a) Preparative Biochem. 18, 443-458). Pertinent data obtained by Ullah is presented in Table 1, below.
The amino acid sequence of the N-terminus of the A. ficuum phytase protein has twice been disclosed by Ullah: Ullah, A. (1987) Enzyme and Engineering conference IX, Oct. 4-8, 1987, Santa Barbara, Calif. (poster presentation); and Ullah, A. (1988b) Prep. Biochem. 18, 459-471. The amino acid sequence data obtained by Ullah is reproduced in FIG. 1A, sequence E, below.
Several interesting observations may be made from the disclosures of Ullah. First of all, the "purified" preparation described in Ullah (1988a and 1988b) consists of two protein bands on SDS-PAGE. We have found, however, that phytase purified from A. ficuum contains a contaminant and that one of the bands found on SDS-PAGE, identified by Ullah as a phytase, is originating from this contaminant.
This difference is also apparent from the amino acid sequencing data published by Ullah (1987, 1988b; compare FIG. 1A, sequences A and B with sequence C). We have determined, in fact, that one of the amino acid sequences of internal peptides of phytase described by Ullah (see FIG. 1B, sequence E) actually belongs to the contaminating 100 kDa protein (FIG. 1C) which is present in the preparation obtained via the procedure as described by Ullah, and seen as one of the two bands on SDS-PAGE (Ullah, 1988a and 1988b). Ullah does not recognize the presence of such a contaminating protein, and instead identifies it as another form of phytase. The presence of such contamination, in turn, increases the difficulty in selecting and isolating the actual nucleotide sequence encoding phytase activity. Furthermore, the presence of the contamination lowers the specific activity value of the protein tested.
Further regarding the sequence published by Ullah, it should be noted that the amino acid residue at position 12, has been disclosed by Ullah to be glycine. We have consistently found using protein and DNA sequencing techniques, that this residue is not a glycine but is in fact a cysteine (see FIGS. 6 and 8).
Finally, Ullah discloses that phytase is an 85 kDa protein, with a molecular weight after deglycosylation of 61.7 kDa (Ullah, 1988b). This number, which is much lower than the earlier reported 76 kDa protein (Ullah, A. and Gibson, D. (1988) Prep. Biochem. 17(1), 63-91) was based on the relative amount of carbohydrates released by hydrolysis, and the apparent molecular weight of the native protein on SDS-PAGE. We have found, however, that glycosylated phytase has a single apparent molecular weight of 85 kDa, while the deglycosylated protein has an apparent molecular weight in the range of 48-56.5 kDa, depending on the degree of deglycosylation.
Mullaney et al. (Filamentous Fungi Conference, April, 1987, Pacific Grove, Calif. (poster presentation) also disclose the characterization of phytase from A. ficuum. However, this report also contains mention of two protein bands on SDS-PAGE, one of 85 kDa, and one of 100 kDa, which were present in the "purified" protein preparation. These protein bands are both identified by the authors as being forms of phytase. A method for transforming microbial hosts is proposed, but has not been reported. The cloning and isolation of the DNA sequence encoding phytase has not been described.
It will be appreciated that an economical procedure for the production of phytase will be of significant benefit to, inter alia, the animal feed industry. One method of producing a more economical phytase would be to use recombinant DNA techniques to raise expression levels of the enzyme in various microorganisms known to produce high levels of expressed peptides or proteins. To date, however, the isolation and cloning of the DNA sequence encoding phytase activity has not been published.